A known measuring instrument of this kind (DE 42 24 253 C1) devised for measuring cylinders of printing presses comprises a carriage which is arranged below and parallel to a bridge of a crane assembly which bridge moves on two rails. Two guide members protrude downwardly from the bridge, and the carriage is guided on them so as to be movable in vertical direction. A distance measuring means which measures the vertical movements of the carriage is arranged outside of the space between the two guide members. The carriage constitutes a transverse beam on which a retaining fixture is movable parallel to the bridge, in other words horizontally. The retaining fixture supports a dual limb bracket in such a way that the latter can be swung back and forth between operative and inoperative positions. When in operative position, the two limbs of the bracket extend vertically downwards so that the bracket forms an inverted U which is open at the bottom. In the inoperative position, the limbs of the bracket extend horizontally and parallel to the transverse base of the bracket. Transmitter and receiver elements, respectively, of a light barrier are arranged at the ends of the two limbs of the bracket. The spacing between these elements is greater than the greatest diameter to be expected of the printing cylinders which are to be measured. A printing cylinder to be measured is placed on a support below the transverse beam of the carriage and parallel to the same so that the bracket which has been pivoted downwards into its operative position will embrace the printing cylinder as the carriage is lowered. The distance by which the carriage is lowered while the printing cylinder interrupts the light barrier is taken as the measure of the printing cylinder diameter.
Likewise known (U.S. Pat. No. 3,555,288) is an instrument for the photoelectric classification of objects, like sections of tree trunks. In this case the objects are movable on a guide means through a U-shaped, stationary rack. The rack comprises two hollow legs extending vertically upwardly from a horizontal transverse member, likewise being hollow, and they each include a vertical guide bar. On each of these two guide bars a carriage is movable up and down. One of these carriages carries a transmitter and the other one a receiver of a horizontal light barrier which is interrupted by the object to be measured. A motor is housed inside the transverse member to serve as the common drive of both carriages. The motor acts through a gear box on two chain drives mounted one each in the two legs of the rack and connected to a respective one of the two carriages.
The measuring accuracies obtainable with the two instruments described above are limited, both because of their structure and their designated purpose.